Chemical plating bath and process



United States Patent CHEMICAL PLATING BATH AND PROCESS Hillard J. Jendrzynski, Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application December 29, 1955 Serial No. 556,068

11 Claims. (Cl. 106-1) This invention relates to nickel plating and more particularly to the chemical deposition or plating of nickel from an aqueous solution of a soluble nickel salt and a suitable reducing agent, such as hypophosphite, a process sometimes referred to as electroless nickel plating. Basically, such deposition involves the reduction of nickel ion to nickel metal by the hypophosphite ion, or probably by the reaction product of the hypophosphite ion with water, the deposition being catalyzed at least initially by the metal of the workpiece and being thereafter autocatalyzed by the nickel metal which is plated. The deposition reaction is represented by the equation:

Electroless nickel plating has found a number of useful applications and it has certain advantages over other methods of plating, such as electrodeposition, chiefly because it provides an extremely uniform plating thickness regardless of the shape of the workpiece; thus uniform, dense, nonporous plate of high quality can be attained on workpieces of highly complex contour. Some of the disadvantages of the electroless nickel plating processes heretofore used have been the relatively slow rate of deposition, that is limitations on plating thickness attainable in a reasonable amount of time, and a relatively high cost principally due to short bath life. One of the particularly troublesome features of electroless nickel plating baths has been the tendency for decomposition, that is, uncontrolled nickel reduction causing a precipitate in the form of a powder can take place. The decomposition is explained by the autocatalytic nature of the plating reaction. As soon as any decomposition takes place to produce nickel metal nuclei, the entire bath decomposes rapidly to produce a precipitate of nickel under the catalytic influence of the first particulate nickel formed. High bath temperatures, while advantageous from the standpoint of increasing the plating rate, have been found to be undesirable in baths heretofore used because of their effect in promoting such decomposition. The problem is aggravated by the fact that locally even higher temperatures may exist due to the presence of heating coils and other such devices. Thus, it is obvious that a substantial contribution would be made to the art if some means were devised to prevent or inhibit decomposition of the plating bath, allowing higher operating temperatures and, therefore, higher plating rates. This would lead to lower production costs due to higher production rates, the improvement of plating efiiciency, and the prevention of loss of valuable chemicals.

It is an object of the present invention to provide an improved electroless nickel plating bath capable of producing thick, dense, nonporous and uniform deposits at a relatively fast rate.

Another object of the invention is the provision of an improved electroless nickel plating process affording a higher rate of deposition and a superior nickel deposit.

More particularly, an object of this invention is to pro- 'ice Bath A Bath B Nickel Sulfate (NlSOt-fiHzO) 16.0 grams-.- 16.0 grams. Sodium Hypophosphite (NaHzPOz-HzO)- 18.0 grams 18.0 grams. Aleqetic Acid (HO H O 12.6 grams..- 124.6 grams. n 0. Molybdic Acid M00 0.020 gram. Water to 1 liter to 1 liter.

If one heats the two baths A and B to a temperature of 205 F., it will be found that bath A will be decomposed within one hour of operation whereas bath B will remain stable indefinitely. In fact, bath B may be further heated to its boiling point (in excess of 212 F.) without decomposing its contents. It can be seen that baths A and B are identical in composition except for the inclusion of a small amount of molybdic acid in bath B. Thus, the molybdic acid serves as a decomposition inhibitor and not only greatly improves the bath for operation at temperatures below or in the neighborhood of about P. which have heretofore been used, but also allows a substantial rise in temperature up to the boiling point without harmful decomposition, thereby providing a much faster plating rate.

In the practice of my invention, any of the commonly known soluble nickel salts may be employed. These include nickel chloride, nickel sulfate, and nickel acetate, or mixtures thereof, nickel sulfate being preferred. The concentration of the nickel ion should preferably be from 1 to 15 g./ 1. The hypophosphite ion may be added in the form of hypophosphorous acid, sodium hypophosphite, or other soluble hypophosphites, the sodium salt being preferred. The hypophosphite ion concentration should preferably be from about 2 to 40 g./l.

An initial pH of about 4.5 to 6.0, which may be attained or adjusted by addition of sodium hydroxide, is preferred. I have obtained optimum results using an initial pH of from 5.0 to 5.6.

In accordance with usual practice, a suitable buffer may be used in order to adjust and maintain the proper bath pH. The amount of buffer used will, of course, depend upon its molecular weight and the number of effective carboxyl groups per molecule since from 1.5 to 2.5 effective carboxyl groups per nickel ion will provide effective buifering. When acetic acid or propionic acid is used as a buffer, from 2 to 30 g./l. are generally sufficient. Heretofore, it has been the practice to use acetic acid or other source of acetate ion as the buffer; however, I prefer to use propionic acid or sodium propionate. Mixtures of propionic and acetic acids, or their soluble salts, may, of course, also be used. Because its pK value is higher, propionic acid is better suited to the preferred pH range of operation than is acetic acid. A slightly smaller amount of propionic acid is required than acetic acid in order to attain the desired buffer action.

In some instances it may be desirable to also include in the bath a chelating agent such as glycine, glycolic acid, ethylene diamine tetra acetic acid and the like, particularly where the nickel ion concentration is high. The

chelating agent functions to maintain the nickel in solution by forming a soluble complex ion with at least a portion of the nickel. Some compounds, for example glycine, serve not only as a chelating agent but also as a buffer to maintain the desired pH.

As stated above, I have found that by far the most advantageous decomposition inhibitor is molybdic acid which may be added as the anhydride, M The commercially available 85% M00 containing amounts, up to of water and possibly ammonia may be advantageously used since it is more readily soluble in the aqueous baths than reagent grade 100% molybdenum trioxide. While the precise reasons are not yet fully known, as a practical matter, I have found that molybdic acid added in amounts up to about 0.035 gram per liter (.02 gram per liter molybdenum), the optimum amount depending upon solution volume as hereinafter more fully explained, is superior chiefly because a more uniform, dense, smooth nickel deposit is obtained.

The optimum concentration of the decomposition inhibitor has been found to be dependent upon volume of plating solution and while the exact nature of this phenomenon is unknown, it suggests a dependence on the surface-volume ratio. The following examples of various baths, for each of which there is listed the optimum molybdic acid concentration for the given bath volume, are illustrative of this phenomenon:

Example III Same as Example I above except:

Bath volume 175 gallons. Molybdic acid (85 M00 0.006-0.009 gram per liter.

Example IV Same as Example I above except: Bath volume 320 gallons. Molybdic acid (85 M00 0.0055-0.006 gram per liter.

Example V Same as Example I above except: Bath volume 640 gallons. Molybdic acid (85% M00 0.0040-00045 gram per liter.

To prepare the plating bath, it is necessary only to dissolve the necessary amount of nickel and hypophosphite salts in water, add the buffer acid, adjust the pH of the resulting solution with sodium hydroxide, dilute to volume, and put in the inhibitors. After heating the resulting solution to an operating temperature of 150 to 210 F., the workpiece is plated by simple immersion. Rates of deposition above 0.0015 inch per hour may be obtained at an operating temperature of 210 F.

The invention may be used either in a continuous type plating bath or in the so-called batch type such as is disclosed in my Patent No. 2,721,814. In the former, there is a continuous or periodic addition of bath ingredients and possibly continuous or periodic removal of undesirable reaction products such as phosphite ion, thereby allowing continued use of the bath over a relatively long period. In the-latter, the bath is used until substantially depleted and is then discarded. The batch type bath is preferable for the practice of the present invention, chiefly because it allows more control over concentrations of ingredients thereby providing a superior nickel deposit. Also, the batch type bath requires simpler and therefore less expensive apparatus thereby effecting a cost saving. Such a bath may be used effectively and efficiently until as high as of the nickel ion originally present is depleted. The depletion of the nickel ion is evidenced by a change in the color of the bath from green to colorless.

Molybdenum compounds other than molybdic acid may be used with beneficial effect in preventing bath decomposition, for example, sodium molybdate. If desired, other additives may be used in addition to the molybdenum compound. In the preferred embodiment, additions of molybdic acid and hydroxylamine sulfate are used. It has been found that the hydroxylamine compound has a noticeable effect in improving the quality of the plate attained.

The following examples of particular baths are additionally illustrative of invention. The total volume of each of the baths is one liter.

Example VI Nickel sulfate (NiSO -6H O) "grams" 16.0 Sodium hypophosphite (NaH PO -H O) do 18.0 Propionic acid dn 10.4 Molybdic acid (85 M00 do 0.020 pH (initial, adjusted with NaOH) 5.2-5.6 Bath temperature boiling Example VII Nickel sulfate grams 16.0 Sodium hypophosphite ....do-.. 18.0 Propionic acid 10.4 Molybdic acid (85% M00 .do 0.015 Phenol do 0.05 pH (initial, adjusted with NaOH) 5.2-5.6 Bath temperature boiling-.. 150

Example VIII Nickel chloride (NiCl -6H O) gra.ms 40.0 Sodium hypophosphite do 10.0 Propionic acid do 20.0 Molybdic acid (85% M00 do-.. 0.020 pH (initial, adjusted with NaOH) 5.0-5.2 Bath temperature "boiling" 150 ExampleIX Nickel acetate [Ni(H -,C O 4H O] grams 10.0 Sodium hypophosphite do 35.0 Molybdic acid (85% M00 do 0.020 pH (initial, adjusted with NaOH) 5.8 Bath temperature "boiling" 150 Example X Nickel acetate grams 30.0 Sodium hypophosphite do 20.0 Hydroxyacetic acid (glycolic acid) do 25.0 Molybdic acid (85% M00 do 0.015 Hydroxylamine sulfate ..do 0.060 pH (initial, adjusted with NaOH) 5.2-5.4 Bath temperature boiling 150 Example XI Nickel chloride grams.. 60.5 Sodium hypophosphite "do"-.. 40.0 Glycine dO Molybdic acid (85% M00 ....do 0.020 pH (initial, adjusted with NaOH) 5.1-5.3 Bath temperature ..-boi1ing 150 It is not essential to the practice of my invention to know the molecular mechanism of its operation, but one can cite certain generally applicable theories. The normal process of producing solids by reaction from homogeneous solution involves, first, the formation of crystal nuclei and, second, the growth of solid material by feeding already existing solid material with further material from the solution. It is normally found that the nucleation process is relatively more difiicult than the growth process in the production of crystals. Illustrations of this may be found in the formation of super-cooled liquids in the freezing of melts and the formation of supersaturated solutions in precipitation processes. In fact, the very existence of the electroless nickel plating process is due to the fact that the growth process on existing solid material takes places in preference to the formation of new nuclei in the solution. The problem in the operation of heretofore existing electroless nickel plating baths has been the narrowness of the range in which satisfactory plating has been obtained without the inducing of the formation of new nuclei. Thus, a plausible explanation of my invention is that the inhibitors serve to widen the range of satisfactory operation by inhibiting the formation of new nuclei.

It is possible to speculate further that the success of my invention lies in the higher degree of adsorption of the inhibitor or the formation of thicker inhibitor films on small particles than on large particles. Support of this view is found in the fact that if a bath is made up with an overly high concentration of inhibitor, it is found that the plating is prevented on corners and edges. The corners and edges of a large piece resemble particles in that they have the common property of having small radii of curvature, and it is suspected that this small radius of curvature is responsible for the higher degree of adsorption or film formation of the inhibitor. In the case of molybdenum oxide as the inhibitor, the inhibiting film apparently has the character of a lower oxide of molybdenum and is observed in the highly inhibited baths as interference films of these oxides, particularly on the corners and edges. It is also possible that the nickel on the corners and edges or in small particles possesses a higher activity and can more readily displace the inhibitor from the solution. Thus, the essential feature of my invention is the replacement of the catalytic or autocatalytic surface in electroless nickel plating, by a noncatalytic surface in a manner such that the noncatalytic surface is distributed preferentially over surfaces of small radii of curvature.

A further guide to the practice of my invention can be found in the related field of catalysis. It is well known that nickel may serve as a catalyst for the hydrogenation of organic compounds. It is also Well known that catalysts of this sort are sensitive to small amounts of inhibiting materials known as catalyst poisons. The essential similarity between electroless nickel plating and hydrogenation catalysis by nickel lies in the receptiveness of the nickel surface for hydrogen and the other chemical substances in a manner that allows the desired reaction to proceed. Thus, the parallelism between the inhibition of electroless nickel plating and the poisoning of nickel hydrogenation catalysts lies in the replacement of the normal catalytic surface by a noncatalytic surface. Functionally, therefore, the inhibitors of my invention might be considered as catalyst poisons in so far as they inhibit the decomposition reaction.

I claim:

1. In an aqueous electroless nickel plating bath containing nickel ion and hypophosphite ion, the improve- I ment which consists of including in said bath a small but effective amount of a molybdenum compound which is soluble in said bath to inhibit the decomposition of said bath while allowing the nickel plating reaction to continue the concentration of said molybdenum compound being such as to provide a molydenum content in said bath of up to .02 gram per liter.

2. In an aqueous electroless nickel plating bath containing nickel ion and hypophosphite ion, the improvement which consists of including in said bath from at least a small but etfective amount up to .035 gram per liter of molybdic acid to inhibit the decomposition of said bath while allowing the nickel plating reaction to continue.

3. An electroless nickel plating bath consisting essentially of an aqueous solution containing from about 1 to 15 grams per liter nickel ion, from about 2 to 40 grams per liter hypophosphite and from at least a small but effective amount up to .035 gram per liter molybdic acid.

4. An electroless nickel plating bath consisting essentially of an aqueous solution containing from about 1 to 15 grams per liter nickel ion, from about 2 to 40 grams per liter hypophosphite and from at least a small but effective amount up to .035 gram per liter molybdic acid, said bath having an initial pH of from 4.5 to 6 and a temperature of from F. to boiling.

5. An electroless nickel plating bath consisting essentially of an aqueous solution containing from about 1 to 15 grams per liter nickel ion, from about 2 to 40 grams per liter hypophosphite, from at least a small but effective amount up to 0.35 gram per liter molybdic acid, and up to .06 gram per liter hydroxylamine sulfate, said bath having an initial pH of from 5 to 5.6 and a temperature of from F. to boiling.

6. An electroless nickel plating bath consisting essentially of an aqueous solution containing about 16 grams per liter nickel sulfate, about 18 grams per liter sodium hypophosphite, about 10.4 grams per liter propionic acid, up to .06 gram per liter hydroxylamine sulfate, and from at least a small but effective amount up to .035 gram per liter molybdic acid, said bath having an initial pH of from 5.0 to 5.6 and a temperature of from 150 F. to boiling.

7. In an electroless nickel plating process, the step of immersing articles to be plated in an aqueous solution maintained at a temperature of from 185 F. to boiling, having an initial pH of from 5.0 to 5.6 and containing from 1 to 15 g./l. nickel ion, from 2 to 40 g./l. hypophosphite ion, from 2 to 30 g./l. of a buffer selected from the group consisting of acetic acid, propionic acid and mixtures thereof, plus from at least a small but effective amount up to .035 g./l. molybdic acid to inhibit decomposition of the solution while allowing the plating reaction to continue.

8. An electroless nickel plating process including the step of immersing the articles to be plated in an aqueous solution maintained at a temperature of from 185 F. to boiling, having an initial pH of from 5.0 to 5.6 and containing about 16 grams per liter nickel sulfate, about 18 grams per liter sodium hypophosphite, about 10.4 grams per liter propionic acid, up to 0.6 gram per liter hydroxylamine sulfate, and from at least a small but effective amount up to .035 gram per liter molybdic acid, said immersion being continued until the nickel ion in said solution is substantially depleted as evidenced by a change in the color of the solution from green to colorless.

9. An electroless nickel plating process comprising the step of immersing articles to be plated in an aqueous solution maintained at a temperature of from 150 F. to boiling, having an initial pH of from 4.5 to 6.0 and containing nickel ion, hypophosphite ion and a small but effective amount of a water soluble molybdenum compound to inhibit decomposition of the solution while allowing the plating reaction to continue, the concentration of said molybdenum compound being such as to provide a molybdenum content in said bath of up to .02 gram per liter.

10. An electroless nickel plating process comprising the step of immersing articles to be plated in an aqueous solution maintained at a temperature of from 150 F. to boiling, having an initial pH of from 4.5 to 6.0 and containing nickel ion, hypophosphite ion and from at least a small but effective amount up to 0.35 gram per liter molybdic acid to inhibit decomposition of the solution while allowing the plating reaction to continue.

11. In an electroless nickel plating process, the step of immersing articles to be plated in an aqueous solution maintained at a temperature of from 150 F. to boiling, having an initial pH of from 4.5 to 6.0 and containing from 1 to 15 g./l. nickel ion, from 2 to 40 g./.l. hypophosphite ion, plus from at least a small but effective amount up to .035 gram per liter molybdic acid to inhibit decomposition of the solution while allowing the plating reaction to continue.

References Cited in the file of this patent UNITED STATES PATENTS 2,694,019 Gutzeit Nov. 9, 1954 FOREIGN PATENTS 515,748 Belgium May 26, 1953 OTHER REFERENCES U. S. National Bureau of Standards, Journal of Research, vol. 39, 1947. Pages 385-395. 

1. IN AN AQUEOUS ELECTROLESS NICKEL PLATING BATH CONTAINING NICKEL ION AND HYPOPHOSPHITE ION, THE IMPROVEMENT WHICH CONSISTS OF INCLUDING IN SAID BATH A SMALL BUT EFFECTIVE AMOUNT OF A MOLYBDENUM COMPOUND WHICH IS SOLUBLE IN SAID BATH TO INHIBIT THE DECOMPOSITION OF SAID BATH WHILE ALLOWING THE NICKEL PLATING REACTION TO CONTINUE THE CONCENTRATION OF SAID MOLYBDEVUM CONTENT IN SAID BATH OF UP TO .02 GRAM PER LITER. 